Building a Series LED Circuit

Wiring two LEDs one after another — and why their voltages add up, not their currents.

You'll need: a supply of at least 6–9V, two LEDs of the same type, one resistor, and a breadboard.

In a series LED circuit, the same current flows through both LEDs one after another, but each one still needs its own share of voltage to light up. That means the voltages add: two red LEDs at roughly 2V each need about 4V just for the LEDs, before the resistor's own voltage drop is added on top. This is why a coin cell (around 3V) usually can't light two series LEDs — there simply isn't enough voltage to go around.

The upside is efficiency: because both LEDs share the same current path, one resistor sets the current for the whole chain, instead of needing a separate resistor per LED. Size that single resistor the same way as Lesson cb-02, just subtract both LEDs' forward voltages from the supply before dividing by your target current.

Keep every LED in the chain facing the same direction — reverse just one, and the whole chain stops conducting.